The electrochemical, chemical or thermal grafting on conducting, semiconducting, or non-conducting materials using aryldiazonium salts is a recognized method for surface functionalization, which has gained tremendous interest over the past decade. The organic layers obtained by this method not only are generally highly stable, much more than those formed by self-assembly chemistry, but also are strongly resistant to heat, chemical degradation and ultrasonication. Furthermore, the method is easy to process and fast (deposition time on the order of 10 s instead of 10-18 h for thiol chemisorption onto Au substrates forming self-assembled monolayers). A major disadvantage is that highly reactive aryl radicals are involved. These attack not only the electrode surface but also already-grafted aryl layers, yielding multi-layers with ramifications, whose structure is, in general, poorly defined. The vertical extension (away from the surface) of the layers is very difficult to control, limiting the possibility of the method in designing complex patterning.
The grafting of material surfaces with aryldiazonium salts has been described in US 2009/0301862, which teaches grafting on non-conducting (insulator) or semi-conducting surfaces, on binary or ternary compounds, and on composite materials. US 2008/0193668 describes forming a film on a support material by chemically grafting aryldiazonium salts. However, grafting with aryldiazonium salts leads to the formation of disorganized multilayers. The formation of monolayers has been proposed through the adequate choice of experimental conditions (including concentration of diazonium salts, applied potential, electrolysis time when electrochemically-driven, solvent . . . ). However, this empirical control of the vertical propagation of diazonium grafting is laborious and, finally, hardly reliable. More recent work reported alternative strategies based on the design of a specific architecture of the diazonium salts. Sterically encumbered substituents on the aryl ring have been exploited to prevent polymerization reactions from taking place, allowing the formation of a near-monolayer of such molecules. A great disadvantage is that the sterically encumbered substituents render these molecules chemically inert thereby precluding any further functionalization.
Other elegant approaches use a formation-degradation sequence. It consists in preparing aryldiazonium salts with a pendant protecting group exhibiting structural or electronic shielding properties. Removal of the protecting group and subsequent post-functionalization allows for attaching functional molecules on the remaining monolayer. These strategies are efficient but require a two-step procedure for the formation of the functionalizable monolayers. Furthermore, void spaces between two adjacent molecules after the deprotection step may be created, being disadvantageous when compact, pinholes-free, layers are required. Very recent work describes a one-step strategy based on the reductive electrografting of a benzene(p-bisdiazonium) salt, leaving a diazonium pendant group for further chemical coupling. However, this approach suffers from a lack of long-term stability of the terminated diazonium layers.
One class of organic substances that has been proposed for immobilizing or grafting onto material surfaces is that of the calix[n]arenes. Calix[n]arenes are cyclic phenoxy derivatives in which n is the number of phenoxy groups, linked in their ortho positions by methylene bridges. Calixarenes are conformationally flexible molecules possessing the ability to undergo complete ring inversions that can display different conformations. Calixarenes can eventually possess a cup-like structure having a narrow and a large rim. Thiacalix[n]arenes are similar to calix[n]arenes except for the bridges that are sulfur ones.
In the following, the term “(thia)calix[n]arenes” refers to both families of compounds, those with methylene bridges (named calix[n]arenes), and those with S bridges (named thiacalix[n]arenes) and their oxide derivatives (SO and SO2 bridges). In addition, (thia)calix[n]arenes also have varying numbers of phenoxy moieties expressed by the symbol [n], wherein n represents the number of phenoxy moieties, in particular n can be 4, 5, or 6. (Thia)calix[n]arenes are known compounds that have been synthesized with various substitution patterns, for example with substituents on the aromatic part of the phenoxy moieties or on its hydroxyls. These cyclic compounds find application in a manifold of areas, including the use as enzyme mimetics, ion sensitive electrodes or sensors, selective membranes, non-linear optics, and in HPLC stationary phases.
(Thia)calix[n]arenes have been used as coatings on various materials. The immobilization of (thia)calixarenes onto a surface has been reported using self-assembly techniques. The resulting immobilized calixarenes were applied as receptors.
US 2003/0228974 describes calixarene derivatives that are immobilized on a metallic or non-metallic oxide surface that has been modified by treatment with a metallic or non-metallic halides, for example silica treated with SiCl4. This approach does not lead to a dense coverage of the metallic or non-metallic oxide surface, and results in branched-off structures, either via the metallic or non-metallic polyhalide link or via the phenolic hydroxyl moieties.
In most of the cases, grafting was achieved by anchoring the small rim of calixarenes onto the substrate. Only rare examples describe grafting via the large rim and, in all cases, it was achieved through thiol chemisorption (Sensors 2007, 7, 1091-1107; Sensors 2007, 7, 2263-2272 and WO 2009/069980).
Yang et al in Angewandte Chem. Intl. Ed. (1996) 35(5) 538-541 describe a calixarene having 4-pyridyl-aldimino arms which covalently bond, through the nitrogen atom of the pyridyl group, to the p-methyl group of a silane coupling agent of a silicone wafer or a fused silica substrate.
There is a need for materials grafted on their surface with a highly robust, structurally regular, ultra-thin layer, which layer preferably is a monomolecular layer. There is a further need for materials grafted with a layer that may serve as a platform for anchoring further molecular entities, which layer may allow spatial pre-organization and pre-structuration as well as the orthogonal polyfunctionalization of the platforms, with a precise spatial control. In addition, there is a need for providing materials grafted with a rather dense (little free surface of the coated material being present) layer. Indeed, for certain applications, it is important to control the surface density, e.g. for applications as sensors, because this may have an impact on the detection sensitivity and efficiency.